This invention relates to mass spectrometers and ion sources therefor. More particularly, this invention is concerned with pulsed ion sources and the provision of a transmission device which gives a pulse ion source many of the characteristics of a continuous source, such that it extends and improves the application of Time of Flight Mass Spectrometry (TOFMS) and that it additionally can be used with a wide variety of other spectrometers, in addition to an orthogonal injection time-of-flight mass spectrometer.
Ion sources for mass spectrometry may be either continuous, such as ESI (electrospray ionization) sources or SIMS (secondary ion mass spectrometry) sources, or pulsed, such as MALDI (matrix-assisted laser desorption/ionization sources). Continuous sources have normally been used to inject ions into most types of mass spectrometer, such as sector instruments, quadrupoles, ion traps and ion cyclotron resonance spectrometers. Recently it has also become possible to inject ions from continuous sources into time-of-flight (TOF) mass spectrometers through the use of xe2x80x9corthogonal injectionxe2x80x9d, whereby the continuous beam is injected orthogonally to the main TOF axis and is converted to the pulsed beam required in the TOF technique. This is most efficiently carried out with the addition of a collisional damping interface between the source and the spectrometer, and this is described in the following paper, having four authors in common with the present invention (Krutchinsky A. N., Chernushevich I. V., Spicer V. L., Ens W., Standing K. G., Journal of the American Society for Mass Spectrometry, 1998, 9, 569-579).
On the other hand, pulsed sources, MALDI sources for example, have usually been coupled directly to TOF mass spectrometers, to take advantage of the discrete or pulse nature of the source. TOF mass spectrometers have several advantages over conventional quadrupole or ion trap mass spectrometers. One advantage is that TOF mass spectrometers can analyze a wider mass-to-charge range than do quadrupole and ion trap mass spectrometers. Another advantage is that TOF mass spectrometers can record all ions simultaneously without scanning, with higher sensitivity than quadrupole and ion trap mass spectrometers. In a quadrupole or other scanning mass spectrometer, only one mass can be transmitted at a time, leading to a duty cycle which may typically be 0.1%, which is low (leading to low sensitivity). A TOP mass spectrometer therefore has a large inherent advantage in sensitivity.
However, TOF mass spectrometers encounter problems with many widely used sources which produce ions with a range of energies and directions. The problems are particularly acute when ions produced by the popular MALDI (matrix-assisted laser desorption/ionization) technique are used. In this method, photon pulses from a laser strike a target and desorb ions whose masses are measured in the mass spectrometer. The target material is composed of a low concentration of analyte molecules, which usually exhibit only moderate photon absorption per molecule, embedded in a solid or liquid matrix consisting of small, highly-absorbing species. The sudden influx of energy is absorbed by the matrix molecules, causing them to vaporize and to produce a small supersonic jet of matrix molecules and ions in which the analyte molecules are entrained. During this ejection process, some of the energy absorbed by the matrix is transferred to the analyte molecules. The analyte molecules are thereby ionized, but without excessive fragmentation, at least in the ideal case.
Because a pulsed laser is normally used, the ions also appear as pulses, facilitating their convenient measurement in a time-of-flight spectrometer. However, the ions acquire a considerable amount of energy in the supersonic jet, with velocities of the order of 700 m/s, and they also may lose energy through collisions with the matrix molecules during acceleration, particularly in high accelerating fields. These and similar effects lead to considerable peak broadening and consequent loss of resolution in a simple linear time-of-flight instrument, where the ions are extracted from the target nearly parallel to the spectrometer axis. A partial solution to the problem is provided by a reflecting spectrometer, which partially corrects for the velocity dispersion, but a more effective technique is the use of delayed extraction, either by itself or in combination with a reflector. In delayed extraction, the ions are allowed to drift for a short period before the accelerating voltage is applied. This technique partially decouples the ion production process from the measurement, making the measurement less sensitive to the detailed pattern of ion desorption and acceleration in any particular case. Even so, successful operation requires careful control of the laser fluence (i.e. the amount of power supplied per unit area) and usually some hunting on the target for a favorable spot. Moreover, the extraction conditions required for optimum performance have some mass dependence; this complicates the calibration procedure and means that the complete range of masses can not be observed with optimum resolution at any given setting. Also, the technique has had limited success in improving the resolution for ions of masses greater than about 20,000 Da. Moreover, it is difficult to obtain high performance MS-MS data in conventional MALDI instruments because ion selection and fragmentation tend to broaden the fragment peak width. The present inventors have realized that these problems can be overcome by abandoning the attempt to maintain the original pulse width, producing instead a quasi-continuous beam with superior characteristics, and then pulsing the injection voltage of the TOF device at an independent repetition rate.
Although coupling to a TOF instrument is used as an example above, problems also arise in coupling MALDI and other pulsed sources to other types of mass spectrometer, such as quadrupole (or other multipole), ion trap, magnetic sector and FTICRMS (Fourier Transform Ion Cyclotron Resonance Mass Spectrometer). Further, it is also desirable to be able to couple MALDI or other pulsed sources to tandem mass spectrometers, e.g. a triple quadrupole or a quadrupole TOF hybrid instrument, which allows MS-MS of MALDI ions to be obtained. Standard MALDI instruments cannot be configured to carry out high performance MS-MS. The dispersion in energy and angle of ions produced by a MALDI source, or similar source, accentuates the difficulty of ion injection. Also, because the residence times of ions in most other types of mass spectrometer are considerably longer than in TOF instruments, the large space charge in the pulse can introduce additional problems. These instruments are all designed to operate with continuous sources, so conversion of the pulsed source to a quasi-continuous one solves most of the problems.
Accordingly, it is desirable to provide an apparatus and method enabling a pulse source, such as a MALDI source, to be coupled to a variety of spectrometer instruments, in a manner which more completely decouples the spectrometer from the source and provides a more continuous ion beam with smaller angular and velocity spreads.
More particularly, it is desirable to provide an improved TOF mass spectrometer with a pulsed ion source, in which the energy spread in the ion beam is reduced, in which the source is more completely decoupled from the spectrometer than in existing instruments, in which problems resulting from ion fragmentation are reduced, enabling new types of measurement, and in which the results obtained from the mass spectrometer and its ease of operation are consequently improved.
It is also desirable to provide a TOF mass spectrometer with both continuous and pulsed sources, for example both ESI and MALDI sources, so either source can be selected.
In accordance with the present invention, there is provided a mass spectrometer system comprising:
a pulsed ion source, for providing pulses of analyte ions
a mass spectrometer;
an ion path extending between the ion source and the mass spectrometer; and
an ion transmission device located in said ion path and having a damping gas in at least a portion of the ion path, whereby there is effected at least one of: a reduction in the energy spread of ions emitted from said ion source; conversion of pulses of ions from the ion source into a quasi-continuous beam of ions; and at least partial suppression of unwanted fragmentation of analyte ions.
The invention has particular applicability to time of flight mass spectrometers. As these require a pulsed beam, conventional teaching is that a pulsed source should be coupled maintaining the pulsed characteristics. However, the present inventors have now realised that there are advantages to, in effect converting a pulsed beam into a continuous, or at least quasi-continuous, beam, and than back into a pulsed beam. The advantages are: improvement in beam quality through collisional damping; decoupling of the ion production from the mass measurement; ability to measure the beam current by single-ion counting because it is converted from a few large pulses to many small pulses, for example from about 1 Hz. to about 4 kHz., or a factor of 4,000; compatibility with a continuous source, such as ESI, offering the possibility of running both sources on one instrument.
The invention also has applicability to mass spectrometers that work with or require a continuous beam. Then, the advantage is that a pulsed source can indeed be used with such spectrometers.
Preferably, the ion source provides the analyte for ionization by radiation, and there is provided a source of electromagnetic radiation, more preferably a pulsed laser, directed at the ion source, for generating radiation pulses to cause desorption and ionization of analyte molecules.
Advantageously, the ion source comprises a target material composed of a matrix and analyte molecules in the matrix, the matrix comprising a species adapted to absorb radiation from the radiation source, to promote desorption and ionization of the analyte molecules.
Preferably, the transmission device comprises a multipole rod set. There can be two or more multipole rod sets and means for supplying different RF and DC voltages to the rod sets.
Collisional damping can also be accomplished in a chamber where no RF field is present providing there is enough buffer gas pressure. In this case ions with reduced velocities can be moved to the exit of the chamber by gas flow drag or a DC electrostatic field. Combinations of electrostatic fields, RF fields and gas flow can also be implemented in a collisional damping chamber.
Another advantage of the invention is that the collisional cooling of the ions helps to reduce the amount of fragmentation of the molecular ions. It is usually desirable to produce a simple mass spectrum containing only ions representative of molecular species. In typical MALDI ion sources, therefore, the laser power must be carefully optimized so that it is dose to the threshold of ionization in order to reduce fragmentation. The inventors have observed, however, that the presence of a gas around the sample surface greatly assists in reducing fragmentation, even at relatively high laser power. Presumably this is due to the effect of collisions with gas molecules which remove internal energy-from the desorbed species before they can fragment. This means that the laser power can be increased in order to improve the ion signal strength, without causing excessive decomposition. The inventors have observed that the amount of fragmentation is decreased as the pressure is increased up to at least approximately 1 torr. Higher pressures may be even more advantageous, but electric fields may be required to avoid clustering reactions at higher pressure.
The mass spectrometer system can include a continuous ion source, and means for selecting one of the pulsed ion source and the continuous ion source, and this then provides the characteristics of two separate instruments in one instrument. The two ion sources can comprise a MALDI source and an ESI source.
Another aspect of the present invention provides a method of generating ions and delivering ions to a mass spectrometer, the method comprising the steps of
(1) providing an ion source;
(2) causing the ion source to produce pulses of ions;
(3) providing an ion transmission device along an ion path extending from the ion source and providing the ion transmission device with a damping gas in at least a portion of the ion path, to effect at least one of a reduction in the energy spread of ions emitted from said ion source; conversion of pulses of ions from the ion source into a quasi-continuous beam of ions; and at least partial suppression of unwanted fragmentation of analyte ions; and
(4) passing ions from the ion transmission device into the mass spectrometer for mass analysis.
The gas pressure of the damping gas can be in the range from about 10xe2x88x924 Torr up to at least 760 Torr. Preferably, step (3) comprises providing an RF rod set within the transmission device. Further, a DC field can be provided between the ion source and the spectrometer to promote movement of ions towards the spectrometer.
The method can include providing two or more rod sets in the ion transmission device, and operating at least one rod set with a DC offset to enable selection of ions with a desired mass-to-charge ratio. A potential difference can be provided between two adjacent rod sets sufficient to accelerate ions into the downstream rod set, to cause collisionally induced dissociation in the downstream rod set.
When a pulsed laser is used, for each laser pulse, a plurality of pulses of ions are delivered into the time-of-flight mass spectrometer.
The ions can first pass through one or more differentially pumped regions that provide a transition from the pressure at the ion source to pressure in the spectrometer. The ion source may be at atmospheric pressure or at least at a pressure substantially higher than that in downstream quadrupole stages and in the mass spectrometer. At least one of these regions can be without any rod set and ion motion towards the mass spectrometer is then driven by gas flow and/or an electrostatic potential.